In many areas of medical diagnosis and therapy, it is desired to selectively deliver an agent, such as a therapeutic agent (a drug) or a diagnostic (e.g. imaging) agent, to a specific site, or a confined region, in the body of a subject such as a patient.
Active targeting of an organ or a tissue is achieved by the direct or indirect conjugation of the desired active moieties (e.g. a contrast enhancing agent or a cytotoxic compound) to a targeting construct, which binds to cell surfaces or promotes cellular uptake at or near the target site of interest. The targeting moieties used to target such agents are typically constructs that have affinity for cell surface targets (e.g., membrane receptors), structural proteins (e.g., amyloid plaques), or intracellular targets (e.g., RNA, DNA, enzymes, cell signaling pathways). These moieties can be antibodies (fragments), proteins, aptamers, oligopeptides, oligonucleotides, oligosaccharides, as well as peptides, peptoids and organic drug compounds known to accumulate at a particular disease or malfunction. Alternatively, a contrast/therapeutic agent may target a metabolic pathway, which is upregulated during a disease (like infection or cancer) such as DNA, protein, and membrane synthesis and carbohydrate uptake. In diseased tissues, abovementioned markers can discriminate diseased cells from healthy tissue and offer unique possibilities for early detection, specific diagnosis and (targeted) therapy.
An important criterion for successful molecular imaging/therapy agents in general and nuclear imaging/therapy agents in particular is that they exhibit a high target uptake while showing a rapid clearance (through renal and/or hepatobiliary systems) from non-target tissue and from the blood. However, this is often problematic: for example, imaging studies in humans have shown that the maximum concentration of a radio labeled antibody at the tumor site is attainable within 24 h but several more days are required before the concentration of the labeled antibody in circulation decreases to levels low enough for successful imaging to take place.
These problems (especially for nuclear imaging and therapy) with slow or insufficient accumulation in target tissue and slow clearance from non-target areas have lead to the application of pre-targeting approaches.
Pretargeting refers to a step in a targeting method, wherein a primary target (e.g. a cell surface) is provided with a Pre-targeting Probe. The latter comprises a secondary target, which will eventually be targeted by a further probe (the Effector Probe) equipped with a secondary targeting moiety.
Thus, in pre-targeting, a Pre-targeting Probe is bound to a primary target. The Pre-targeting Probe also carries secondary targets, which facilitate specific conjugation to a diagnostic (imaging) and/or therapeutic agent, the Effector Probe. After the construct forming the Pre-targeting Probe has localized at the target site (taking time, e.g. 24 h), a clearing agent can be used to remove excess from the blood, if natural clearance is not sufficient. In a second incubation step (preferably taking a shorter time, e.g., 1-6 hours), the Effector Probe binds to the (pre)bound Pre-targeting Probe via its secondary targeting moiety. The secondary target (present on the Pre-targeting Probe) and the secondary targeting moiety (present on the Effector Probe) should bind rapidly, with high specificity and high affinity and should be stable within the body.
The general concept of pre-targeting is outlined for imaging in FIG. 1. Herein the Effector Probe is an imaging probe comprising a detectable label for an imaging modality. The Effector Probe binds to the (pre)-bound Pre-targeting Probe via its secondary targeting groups.
Common examples for secondary target/secondary targeting moiety pairs are biotin/streptavidin or antibody/antigen systems. To be effective, the Effector Probe must be rapidly excreted from the body (e.g., through the kidneys) to provide the desired high tumor accumulation with relatively low non-target accumulation. Therefore, these probes are usually small.
In nuclear imaging and radiotherapy the concept of pre-targeting is of further advantage, as the time consuming pre-targeting step can be carried out without using radionuclides, while the secondary targeting step using a radionuclide can be carried out faster. The latter allows the use of shorter lived radionuclides with the advantage of minimizing the radiation dose to the patient and, for instance, the usage of PET agents instead of SPECT agents. Using a pre-targeting approach in MRI in combination with multidentate ligand systems (streptavidin, dendrimers) can afford signal amplification at target sites. Furthermore, in general, this approach facilitates the usage of a universal contrast agent.
The entities that carry out highly selective interactions in biology in general (like antibody-antigen), and in pre-targeting in particular (biotin-streptavidin, antibody/haptens, antisense oligonucleotides), are very large. As a result, pre-targeting with peptides and small organic moieties as primary targeting groups, as well as metabolic imaging and intracellular target imaging, have remained out of reach as the size of the secondary targets makes the use of small primary groups pointless.
Moreover, the current pretargeting systems are hampered by factors associated with their biological nature. Biotin is an endogenous molecule and its conjugates can be cleaved by the serum enzyme biotinidase. When antisense pre-targeting is used, the oligonucleotides can be subject to attack by RNAse and DNAse. Proteins and peptides are also subject to natural decomposition pathways. These interactions can be further impaired by their non-covalent and dynamic nature and limited on-target residence time. Also, endogenous biotin competes with biotin conjugates for streptavidin binding. Finally, streptavidin is highly immunogenic.
A recent development is to avoid the drawbacks associated with pretargeting solely on the basis of natural/biological targeting constructs (i.e., biotin/streptavidin, antibody/hapten, antisense oligonucleotides).
A reference in this respect is WO 2010/051530, wherein pretargeting is discussed on the basis of the reactivity between certain dienes, such as tetrazines and dienophiles such as a trans-cyclooctenol (TCO).
Although on the basis of such systems a relatively fast reaction can be obtained, this does not come near the reactivity of the above-mentioned biotin-streptavidin system. Hence, avoiding the drawbacks of the latter, goes at cost of the primary requirement of the reaction, viz. speed. It is thus desired to provide a system that is not based on biomolecules as discussed above, and yet has a desirably fast reaction rate.